Shape Memory Alloys

Theresa Valentine

ENMA490

Fall 2002

Shape Memory Effect

Martensite-austenite transformation Austenite is parent, high-temperature phase,

cubic structure Martensite is low-temp phase, usually tetragonal [A] [twinned M] on cooling, diffusionless

shear transformation Deformation of martensite moves twin

boundaries; recovered on heating and transformation to austenite

Shape Memory Effect

Shape memory effect mechanism, showing (a) undeformed parent crystal, (b) martensite, (c and d) deformed martensite through twin boundary movement, and (e) reversion to the parent phase after heating. From Otsuka (1998), p.37, fig. 2.11.

Shape Memory Effect

Free-energy versus temperature curves for the parent (Gp) and martensite (Gm) structures in a shape memory alloy. From Otsuka (1998), p.23, fig. 1.17.

Martensite-austenite phase transformation in shape memory alloys. From http://www.tiniaerospace.com/sma.html.

Superelasticity

Stress-induced martensite formation above transition temperature Martensite immediately reverts to austenite once stress is

removed Large recoverable deformation

From http://www.sma-inc.com/SMAandSE.html

Nickel-Titanium

Near-equiatomic NiTi most widely used SMA todayProperty Value

Transformation temperature -200 to 110 C

Latent heat of transformation 5.78 cal/g

Melting point 1300 C

Specific heat 0.20 cal/g

Young’s modulus 83 GPa austenite; 28 to 41 GPa martensite

Yield strength 195 to 690 MPa austenite; 70 to 140 MPa martensite

Ultimate tensile strength 895 MPa annealed; 1900 MPa work-hardened

% Elongation at failure 25 to 50% annealed; 5 to 10% work-hardened

From http://www.sma-inc.com/NiTiProperties.html

Nickel-Titanium

B2 (cesium chloride) crystal structure. From http://cst-www.nrl.navy.mil/ lattice/struk/b2.html

B19’ crystal structure. From Tang et al., p.3460, fig.5.

Parent β (austenite) phase with B2 structure

Martensite phase with monoclinic B19’ structure

Nickel-Titanium

Intermediate R phase can nucleate in B2, then B19’ phase grows from R

1 and 2 show single dislocations in B2 from which an R phase grows

Nucleation of R-phase in an alloy of Ti-48Ni-2Al from dislocations. From Otsuka (1998), p.56, fig. 3.7.

Shape Memory Alloys Today

Shape memory effect means deforming at low temperature, changing back at high temperature

Shape memory alloys (SMAs) first discovered 1951 NiTi SMA discovered 1963 Macroscale applications as:

Tube couplings Air-directing flaps Spring actuators

Macroscale SMAs

“Magic flower”http://www.kobico.co.kr/english/ek3.html

Macroscale SMAs

“Climbing koala”http://www.kobico.co.kr/english/ek3.html

Macroscale SMAs

Eyeglass frames – superelastic NiTihttp://www.flexon.com/HTML2001/flexon01.html

MEMS Applications for SMAs

TiNi pneumatic microvalvehttp://www.sma-mems.com

MEMS Applications for SMAs

NiTi microbubble (UCLA)http://aml.seas.ucla.edu/

MEMS Applications for SMAs

Flow controlhttp://www.afrlhorizons.com/Briefs/Sept02/OSR0203.html

MEMS Applications for SMAs

Surgical micro-wrapperhttp://www.afrlhorizons.com/Briefs/Sept02/OSR0203.html

MEMS Applications for SMAs

One-axis translation stagehttp://dmtwww.epfl.ch/isr/hpr/sma.html

MEMS Applications for SMAs

Micro-gripperhttp://dmtwww.epfl.ch/isr/hpr/sma.html

Future Research

Thin films for micro-actuators, micro-devices Compositional changes during sputtering Accurate phase diagram necessary

Two-way shape memory effect Alloying NiTi with small percentages of other

metals (Cu, Fe)